COEXISTING PHASES AND CRITICALITY IN NACL BY COMPUTER-SIMULATION

The liquid-vapor coexistence curve of NaCl is evaluated by molecular dynamics simulations in using the well-known Born-Huggins-Mayer-Fumi-Tosi (BHMFT) interionic potential. Due to the limited size of our sample (N-ions=512), the calculated isotherms present van der Waals loops which can be described by an empirical equation of state whose critical parameters are T-c=3068 K, p(c)=0.174 g/cm(3), and rho(c)-105.4 bar. By comparing with the available experimental data on molten NaCl and according to corresponding states arguments, we an able to deduce from our simulation data an estimation of the critical parameters of real NaCl, namely, T-c=3300 K, p(c)=O. is g/cm(3), and rho(c)=325 bar. The electrical conductivity of our simulated molten salt is then evaluated along the coexistence curve, in the highly compressed liquid and in the dilute gas. No evidence for an insulator-ionic conductor transition is found. Instead a continuous transition between a highly conducting fluid at high density and a low conducting gas at low temperature is pointed out. The degree of dissociation is obtained from the conductivity data and from the knowledge of the self-diffusion coefficients through a generalized Nernst-Einstein relation. The evolution of the degree of dissociation with density and temperature is corroborated by the analysis of the charge-charge distribution functions which show the occurrence of ion pairing at low density and low temperature. Finally, the mean field type behavior exhibited by our simulated molten salt is discussed in perspective with the available experimental data for other fused salts as well as with the current state of the theory concerning the criticality in ionic fluids.